1. Field of the Invention
This invention relates to position monitoring systems, and more particularly, to systems for tracking relative movement between a light beam and a grating.
2. Description of the Prior Art
Systems for tracking relative movement between a light beam and a grating are well known. For example, a pattern generator in which a portion of a moving laser beam is directed onto a stationary grating and light from the beam passing through the grating is sensed by a photodetector is illustrated in FIG. 1 of an article by M. J. Cowan et al. entitled "The Primary Pattern Generator Part I -- Optical Design," beginning on page 2033 in the issue of the Bell System Technical Journal dated November 1970. The grating in this example is alternatively called a code plate. The position of the laser beam is tracked by counting interruptions in the laser beam. A similar system is disclosed in copending application Ser. No. 466,313 of V. J. Zaleckas, filed May 2, 1974, and now U.S. Pat. No. 3,902,036 and assigned to the assignee of this invention, wherein a patterned laser beam is deflected by mirrors rotated by galvanometers. Portions of the pattern are laterally spaced so that one portion is blocked by a line in a grating when the other portion is passed by the grating. Using two photodetectors, two out-of-phase signals are then obtained from which the direction of relative motion between the beam and the grating can be determined.
Systems wherein a grating moves with respect to a stationary light beam are also known. For example, automated machine tools have been built having a grating mounted on a movable worktable and a light beam and a photodetector mounted on a stationary bed, for use in tracking the position of the bed. Again, the beam can be patterned and two photodetectors used to generate out-of-phase signals from which the direction of relative movement can be obtained.
The above systems depend on a light beam being either blocked or passed by the lines in the grating. The light beam must be small enough in diameter in the plane of the grating so that it can be blocked by a grating line, or passed by the space between grating lines. These lines may be spaced as closely as 20.mu.m apart. Defects in a grating or dust on the grating can thus affect the passage of the light beam and cause inaccuracies in the recorded position.
The precision of the above systems is a function of the spacing between the grating lines, and can only be increased by reducing that spacing and reducing the diameter of the incident light beam. There are practical limits to the minimum spacing of grating lines, and these limits prevent more precise systems from being built using the above techniques.
What is desired for use in such systems as those described above is a system for tracking the relative positions of a light beam and a grating that is relatively insensitive to defects in the grating or dust on the grating, and that is capable of higher precision with a given prior art grating.